Introduction:

corrensite is a clay mineral consisting of regular interstratifications of trioctahedral chlorite and trioctahedral smectite in equal proportions. It most typically develops as an authigenic mineral (one that forms after deposition) in sedimentary rocks such as mudstone and shales, or as an alteration product in hydrothermally altered rocks, particularly those of igneous origin.

Occurrence in Wales:

Merriman & Roberts (1985) described the occurrence of corrensite in rocks of Arenig and Llanvirn (Ordovician) age from Snowdonia and Llŷn, in mudstones close to intrusions and which had been affected by contact metamorphism prior to regional metamorphism. On Llŷn, chlorite/vermiculite is associated with basic intrusions in the Rhiw area, and chlorite/smectite is associated with intrusions in the Llanbedrog and Yr Eifl areas. Roberts & Merriman (1990) have also identified corrensite as a component of metabentonites of Middle Cambrian age from the St. Tudwal's Peninsula, also on Llŷn. More recently Garvie & Metcalfe (1997) recorded corrensite from veins cutting Ordovician volcanic rocks at Builth Wells.

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